You could use that but it's not as good as using a short description of the picture itself.  The alt tags are what Google uses to determine what the picture is of since they can't actually see the image.  If you use keyword optimized alt tags that are natural not keyword stuffed then you will probably also bring in more Google image traffic.

Hope this helps,

You should setup rel=canonical tags to be automatically added to each of the affiliate pages that are duplicates of your main page.  This basically tells Google that your original URL without affiliate parameters is the authoritative version of the content and that all other pages should not be indexed above the canonical version.

You should also probably add a query string to tell Google Webmaster Tools to ignore URL's that contain your affiliate query parameter.  This will make sure that Webmaster Tools is setup correctly so you aren't getting hit for duplicate content.

Hope this helps!

The only metadata that the search engines look at are image titles and alt tags.

of you are referring to all of the metadata stored in a RAW image then no the engines don't use this data today That we know of.  As seo morphs more into content and content becomes more and more visual this isn't to say that they might not use it in the future.

In my opinion CL is great for the traffic it can drive but not good for link equity.  I'm pretty sure they nofollow all of the links from Craigslist so the only real value is the traffic.  If the listings are expired and still cached by craigslist I would suggest seeing if you can't login to craiglist to manage the ads either by adding new ones or possibly deleting old ones.

I agree with both responses, there is no reason to 301 redirect just because of new title tags.  I think 3 redirects is really pushing it in terms of retaining your link value but 2 I have done several times with no effects on rankings.

Hope this helps!